1. Field of the Invention
The present invention relates to an apparatus used in a vertical type flexible package forming, filling and closing machine to heat-seal the side edges of a cylindrically formed package web in a feeding direction of the web.
2. Description of the Related Art
Vertical type flexible package forming, filling and closing machines are generally associated with a pinch roller type sealing apparatus as disclosed in Japanese Utility Model Registration No. 2528263, or with a seal belt type sealing apparatus as disclosed in Unexamined Japanese Patent Publication No. 8-318905. The former type sealing apparatus includes two pairs of pinch rollers and a center sealer. The two pairs of pinch rollers are spaced from each other in a vertical direction along a filling tube, and the center sealer is arranged between the upper and lower pairs of pinch rollers. A package web is formed into cylindrical shape on the outer periphery of the filling tube, with side edges thereof forming a joint where the inner faces of the side edges come into plane contact with each other. The joint of the web passes through the center sealer while being held by the upper and lower pairs of pinch rollers, and at this time, the joint is heat-sealed, that is, vertically sealed.
On the other hand, the latter type sealing apparatus has an endless seal belt extending along a filling tube. A package web formed into cylindrical shape on the outer periphery of the filling tube has a joint where side edges thereof overlap each other. The web is fed with its joint held between the seal belt and the filling tube, and in this process the joint is heat-sealed, that is, vertically sealed.
In the former type sealing apparatus, since the joint of the package web passes through the center sealer while being held by the upper and lower pairs of pinch rollers, the sealed joint is inevitably in the form of fin seal and it is not possible to form a lap seal.
With the latter type sealing apparatus including a seal belt, on the other hand, the joint can be sealed in either form of fin seal or lap seal. However, since the joint of the package web is heat-sealed by heat transmitted thereto from a heater block through the medium of the seal belt, it is difficult to control the application of heat to the joint. To make up for a deficiency of heat applied to the joint, therefore, the heating temperature of the heater block needs to be set at high temperature. Also, the seal belt and a drive source therefor make the structure of the vertical sealing apparatus complicated.